This invention relates to a high rigidity glass-ceramic substrate and, more particularly, to a high rigidity glass-ceramic substrate for a magnetic information storage medium used, e.g., for a magnetic information storage device having a super flat substrate surface capable of coping with near contact recording or contact recording employed in the ramp loading system and capable also of coping with a high speed rotation of a magnetic information storage medium. The invention relates also to a magnetic information storage medium such as a magnetic disk which is provided by forming a film of an information storage medium on the glass-ceramic substrate.
In this specification, the term xe2x80x9cmagnetic information storage mediumxe2x80x9d means a magnetic information storage medium in the form of a disk and includes fixed type hard disks, removable type hard disks and card type hard disks used respectively for so-called xe2x80x9chard disksxe2x80x9d for personal computers and also other disk type magnetic information storage media which can be used in HDTV, digital video cameras, digital cameras, mobile communication devices etc. In this specification, the term xe2x80x9cspinerxe2x80x9d means at least one of (Mg and/or Zn)Al2O4, (Mg and/or Zn)2TiO4 and a mixture in the form of a solid solution between these two crystals and the term xe2x80x9cspinel solid solutionxe2x80x9d means a crystal in which other ingredient is mixed with spinel and/or a part of spinel is substituted by other ingredient.
Recent development of personal computers for multi-media purposes and digital video cameras and digital cameras requires handling of a large amount of data such as a moving picture and voice and, for this purpose, a magnetic information storage device of a large recording capacity is required. For increasing the recording density, there is a tendency in the art of a magnetic information storage medium toward reducing the size of a bit cell and thereby increasing the bit and track density. As a result, the magnetic head performs its operation in closer proximity to the disk surface. For coping with starting and stopping of a magnetic head which is operated in a near contact state or a contact state with respect to a magnetic information storage medium, a landing zone system has been developed according to which a specific part (an unrecorded part in a radially inside or outside portion of a disk) is processed for preventing stiction of the magnetic head to the disk.
In the current magnetic information storage device, the CSS (contact start stop) system is adopted according to which (1) the magnetic head is in contact with the magnetic information storage medium before starting its operation and, (2) when the magnetic head has started its operation, the magnetic head flies over the surface of the magnetic information storage medium. If the plane of contact between the magnetic head and the magnetic information storage medium is exceedingly in the state of a mirror surface, stiction of the magnetic head occurs resulting in unsmooth starting of rotation due to increased friction and damage to the surface of the magnetic information storage medium or the magnetic head. Thus, the magnetic information storage medium faces conflicting demands for lower flying height of the magnetic head accompanying increase in the storage capacity and prevention of stiction of the magnetic head to the surface of the magnetic information storage medium. As an answer to satisfy such conflicting demands, a ramp loading technique has been developed according to which the magnetic head in operation is completely in contact with the surface of the magnetic information storage medium but starting and stopping of the magnetic head are performed in an area outside of the surface of the magnetic information storage medium. Thus, there has been an increasing demand for a smoother surface of the magnetic information storage medium.
Developments are also in progress for transferring information at a higher speed by rotating a magnetic information storage medium at a higher speed. Since, however, a high speed rotation of a substrate for the magnetic information storage medium causes deflection and deformation in the substrate, the substrate is required to have a higher Young""s modulus. Further, in addition to the currently used fixed type hard disks, magnetic information storage devices which use removable type hard disks and card type hard disks which require a high strength of the substrate are being considered and becoming feasible and application of the substrate to HDTV, digital video cameras, digital cameras and mobile communication devices is under way.
In the situation in which a high rigidity substrate material is required, an aluminum alloy substrate cannot provide a sufficient strength and, if thickness of the substrate is increased, it will make it difficult to realize a compact and light-weight design of the medium. For solving the problem inherent in the aluminum alloy substrate, known in the art are chemically tempered glasses such as alumino-silicate (SiO2xe2x80x94Al2O3xe2x80x94Na2O) glass proposed by Japanese Patent Application Laid-open Publication Nos. Hei 8-48537 and Hei 5-32431 etc.). These chemically tempered glasses, however, have the following disadvantages: (1) Since polishing is made after the chemical tempering process, the chemically tempered layer is seriously instable in making the disk thinner. (2) Since the chemically tempered phase causes aging when used for a long time, magnetic properties of the magnetic information storage medium are deteriorated. (3) Since the glass contains Na2O or K2O ingredient as an essential ingredient, these alkali ingredients diffuse in the film formed during the film forming process and thereby deteriorate magnetic properties of the magnetic information storage medium. For preventing this, a barrier coating over the entire surface for preventing diffusion of Na2O or K2O becomes necessary and this makes it difficult to produce the product in a stable manner at a low cost. (4) Chemical tempering is made for improving mechanical strength of the glass but this is based on utilization of tempering stress between the surface phase and the inside phase. Young""s modulus of the chemically tempered glass is 830 GPa or below which is equivalent to ordinary amorphous glass and this poses limitation to application of the chemically tempered glass to a high speed rotation drive. Thus, the chemically tempered glasses are not sufficient as a substrate for a high recording density magnetic information storage medium.
Aside from the aluminum alloy substrates and chemically tempered substrates, known in the art are some glass-ceramic substrates. For example, glass-ceramic substrates disclosed in Japanese Patent Application Laid-open Publication No. Hei 9-35234 and EP0781731A1 have a Li2Oxe2x80x94SiO2 composition and contain lithium disilicate and xcex2-spodumene crystal phases or lithium disilicate and xcex2-cristobalite crystal phases. In the glass-ceramic substrates, however, no consideration or suggestion is made about relation between Young""s modulus and specific gravity al all. Young""s modulus of these glass-ceramics is 100 GPa at the maximum.
For improving such low Young""s modulus, Japanese Patent Application Laid-open Publication No. Hei 9-77531 discloses a glass-ceramic of a SiO2xe2x80x94Al2O3xe2x88x92Mgoxe2x80x94ZnOxe2x80x94TiO2 system and a rigid disk substrate for a magnetic information storage medium. This glass-ceramic contains a large quantity of spinel as a predominant crystal phase and also contains MgTi2O5 and other crystal phases as sub-crystal phases and has Young""s modulus of 96.5-165.5 GPa. In this material, the predominant crystal phase is only spinel represented by (Mg/Zn)Al2O4 and/or (Mg/Zn)2TiO4 and the sub-crystal phases are not limited to specific crystals but crystals of a broad range are mentioned. Further, this glass-ceramic contains a large amount of Al2O3 and is different from the glass-ceramics of the present invention which contain a relatively small amount of Al2O3 and have a high Young""s modulus and resistivity to devitrification. Such large amount of Al2O3 deteriorates melting property of a base glass and resistivity to devitrification and thereby adversely affects productivity. The proposed glass-ceramic, therefore, is a merely rigid material. Furthermore, the glass-ceramic of this system has an exceedingly high surface hardness (Vickers hardness) and this adversely affects processability and large scale production. Accordingly, the improvement achieved by this substrate material is still insufficient for a substrate of a high recording density magnetic information storage medium.
WO 98/22405 publication discloses a glass-ceramic of a SiO2xe2x80x94Al2O3xe2x80x94MgOxe2x80x94ZrO2xe2x80x94TiO2xe2x80x94Li2O system. This glass-ceramic contains xcex2-quarts solid solution as a predominant crystal phase and enstatite, spinel and other crystals as sub-crystal phases and has a crystal grain diameter of 1000 xc3x85 or below. This glass-ceramic, however, requires Li2O as an essential ingredient in its composition and, besides, requires xcex2-quarts solid solution as its predominant crystal phase and, therefore, is entirely different from the glass-ceramics of the present invention.
It is, therefore, an object of the present invention to overcome the problems of the prior art substrates and provide a high rigidity glass-ceramic substrate which is suitable for a substrate of a magnetic information storage medium having excellent surface characteristics capable of coping with the ramp loading system (i.e., contact recording of the magnetic head) for high density recording and having a high Young""s modulus characteristics capable of coping with a high speed rotation and surface hardness characteristics suitable for processing.
It is another object of the invention to provide a magnetic information storage disk provided by forming a magnetic information storage film on such glass-ceramic substrate.
Accumulated studies and experiments made by the inventors of the present invention for achieving the above described objects of the invention have resulted in the finding, which has led to the present invention, that a high rigidity glass-ceramic substrate can be provided which contains, as a predominant crystal phase or phases, at least one crystal phase selected from the group consisting of enstatite (MgSiO3), enstatite solid solution (MgSiO3 solid solution), magnesium titanate (MgTi2O5) and magnesium titanate solid solution (MgTi2O5 solid solution), has fine crystal grains (preferably globular crystal grains) of precipitated crystal phases, has excellent melting property of a base glass, high resistivity to devitrification, easiness in polishing, excellent smoothness in the surface after polishing and has high Young""s modulus capable of coping with a high speed rotation.
For achieving the objects of the invention, there is provided a high rigidity glass-ceramic substrate having a predominant crystal phase or phases, said predominant crystal phase being at least one selected from the group consisting of enstatite (MgSiO3), enstatite solid solution (MgSiO3 solid solution), magnesium titanate (MgTi2O5) and magnesium titanate solid solution (MgTi2O5 solid solution), and glass-ceramic constituting the glass-ceramic substrate having Young""s modulus within a range from 115 GPa to 160 GPa and containing less than 20 weight percent of Al2O3 ingredient.
In one aspect of the invention, there is provided a high rigidity glass-ceramic substrate having a predominant crystal phase or phases, said predominant crystal phase comprising enstatite (MgSiO3) or enstatite solid solution (MgSiO3 solid solution) as a first crystal phase having the largest ratio of precipitation, and glass-ceramic constituting the glass-ceramic substrate having Young""s modulus within a range from 115 GPa to 160 GPa and containing less than 20 weight percent of Al2O3 ingredient.
In another aspect of the invention, there is provided a high rigidity glass-ceramic substrate having a predominant crystal phase or phases, said predominant crystal phase comprising magnesium titanate (MgTi2O5) or magnesium titanate solid solution (MgTi2O5 solid solution) as a first crystal phase having the largest ratio of precipitation, and glass-ceramic constituting the glass-ceramic substrate having Young""s modulus within a range from 115 GPa to 160 GPa and containing less than 20 weight percent of Al2O3 ingredient.
In another aspect of the invention, there is provided a high rigidity glass-ceramic substrate having a predominant crystal phase or phases, said predominant crystal phase comprising enstatite (MgSiO3) or enstatite solid solution (MgSiO3 solid solution) as a first crystal phase having the largest ratio of precipitation and at least one selected from the group consisting of magnesium titanate (MgTi2O5), magnesium titanate solid solution (MgTi2O5 solid solution), spinel and spinel solid solution as a second crystal phase having a smaller ratio of precipitation than the first crystal phase, and glass-ceramic constituting the glass-ceramic substrate having Young""s modulus within a range from 115 GPa to 160 GPa and containing less than 20 weight percent of Al2O3 ingredient.
In another aspect of the invention, there is provided a high rigidity glass-ceramic substrate having a predominant crystal phase or phases, said predominant crystal phase comprising magnesium titanate (MgTi2O5) or magnesium titanate solid solution (MgTi2O5) as a first crystal phase having the largest ratio of precipitation and at least one selected from the group consisting of enstatite (MgSiO3) or enstatite solid solution (MgSiO3 solid solution), spinel and spinel solid solution as a second crystal phase having a smaller ratio of precipitation than the first crystal phase, and glass-ceramic constituting the glass-ceramic substrate having Young""s modulus within a range from 115 GPa to 160 GPa and containing less than 20 weight percent of Al2O3 ingredient.
In another aspect of the invention, said glass-ceramic is substantially free from Li2O, Na2O and K2O.
In another aspect of the invention, the high rigidity glass-ceramic substrate has a surface roughness Ra (arithmetic mean roughness) after polishing of 8 xc3x85 or below and a maximum surface roughness Rmax after polishing of 100 xc3x85 or below.
In another aspect of the invention, the high rigidity glass-ceramic substrate has a coefficient of thermal expansion within a range from 40xc3x9710xe2x88x927/xc2x0 C. to 60xc3x9710xe2x88x927/xc2x0 C. within a temperature range from xe2x88x9250xc2x0 C. to +70xc2x0 C.
In another aspect of the invention, said predominant crystal phase has a crystal grain diameter within a range from 0.05 xcexcm to 0.30 xcexcm.
In another aspect of the invention, the high rigidity glass-ceramic substrate has Vickers hardness within a range from 6860 N/mm2 to 8330 N/mm2.
In another aspect of the invention, the glass-ceramic comprises in weight percent on the oxide basis:
In another aspect of the invention, the high rigidity glass-ceramic substrate further comprises an element selected from the group consisting of P, W, Nb, La, Y and Pb in an amount up to 3 weight percent on the oxide basis and/or an element selected from the group consisting of Cu, Co, Fe, Mn, Cr, Sn and V in an amount up to 2 weight percent on the oxide basis.
In another aspect of the invention, the high rigidity glass-ceramic substrate is provided by melting glass materials, forming and annealing a base glass and subjecting the base glass to heat treatment for crystallization under a nucleation temperature within a range from 650xc2x0 C. to 750xc2x0 C., a nucleation time within a range from one hour to twelve hours, a crystallization temperature within a range from 850xc2x0 C. to 1000xc2x0 C. and a crystallization time within a range from one hour to twelve hours.
In still another aspect of the invention, there is provided a magnetic information storage disk provided by forming a film of a magnetic information storage medium on the above described high rigidity glass-ceramic substrate.